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On the failure of single-story wood-framed houses in severe storms
Journal of Wind Engineering and Industrial Aerodynamics, 29 (1988) 245-252
245
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
ON THE FAILURE OF SINGLE-STORY WOOD-FRAMEDHOUSES IN SEVERE STORMS P.R. SPARKS, M.L. HESSIG, J.A. MURDENand B.L. SILL Department of Civil Engineering, Clemson Univerity, Clemson, South Carolina 29634-0911, (USA)
ABSTRACT Wind-tunnel pressure measurements are used to define the wind forces at the roof to wall interface of typical single-story, wood-framed houses. These forces are then combined with the weight of the roof, connector capacities and the effects of internal pressure to estimate the most l i k e l y wind speeds at which a roof would separate from the walls. This is then extended to predict the risk of serious structural damage in a typical design hurricane, taking into account the form of construction, amount of shelter and the degree to which the windows are protected. Conclusions are drawn regarding the s u i t a b i l i t y of damage observations to estimate wind speeds and the s u s c e p t i b i l i t y of various forms of construction to damage in severe storms. INTRODUCTION Annual losses due to wind damage in the United States are estimated to be in excess of 3 b i l l i o n dollars (ref. 1).
A significant proportion of this damage
occurs in single-story, wood-framed houses during severe thunderstorms, tornadoes and hurricanes.
Damage is so common that Meteorologists have
developed a scale, the Fujita Scale, which classifies the extent of damage and then estimates wind speeds from that c l a s s i f i c a t i o n .
Unfortunately, t h i s scale
has been used extensively without ever being properly calibrated.
The
assumption is made that the amount of damage bears a linear relationship to the i n t e n s i t y of the storm, despite the fact that wood-framed dwellings are very b r i t t l e structures in which catastrophic f a i l u r e s often occur shortly after the loss of a roof. Modern roof construction, using trusses and plywood, usually produces a very sound structural unit which may separate intact, or at least in large sections, from the walls.
The c r i t i c a l connection, upon which the survival of the
complete structure may depend, is therefore that between the roof trusses or j o i s t s and the stud-walls. This paper describes the use of wind-tunnel test data to determine the roof-wall interface forces on a set of typical single-story houses with a variety of roof shapes, each 24 f t . by 48 f t .
(7.3m x 14.6m) in plan with 10 f t .
(3.0m) high walls and 2 f t . (O.6m) eaves. This information is then used to 0167-6105/88/$03.50
© 1988 Elsevier Science Publishers B.V.
246
assess the wind speeds at which roof failures are l i k e l y to occur and to determine the v u l n e r a b i l i t y of existing buildings to a typical design hurricane. WIND-TUNNEL TESTS These tests were conducted in the Boundary Layer Wind Tunnel at Clemson University.
Since most damagetakes place in buildings located in open country,
a boundary layer was developed to model this situation.
The mean velocity
p r o f i l e had a power law exponent of 0.16 and the longitudinal turbulence intensity was 18%. Using 1:48 scale models with typical roof slopes of 4:12 (18°), 6:12 (27°), 8:12 (34° ) and 12:12 (45° ) (Fig. 1), pressures were measured at 36 locations on the gable roofs and 54 locations on the hip roofs.
Since the turbulence
characteristics of most severe storms are not well established, i t was decided for this investigation not to attempt to use a sophisticated simultaneous pressure measurement technique.
Instead, the more t r a d i t i o n a l approach of
measuring mean values and combining with gust wind speeds, averaged over an appropriate time (3 seconds), was adopted. Outputs from pressure transducers were recorded on a microcomputer and then converted to force and moment coefficients with respect to axes passing through the center of the plane of intersection of the walls and the roof.
Further details of the wind-tunnel
tests can be found in reference 2. WIND CONDITIONS REQUIREDTO PRODUCEROOFFAILURES The results from the wind-tunnel tests were combined with roof weight and the forces resulting from internal pressure to determine roof-wall interface forces.
In doing this i t was assumed that connections would only be made on the
exterior walls parallel to the roof ridge.
This was appropriate for gable roofs
but s l i g h t l y conservative for hip roofs where some connections would be made on the end walls. For most roofs the greatest interface forces occurred at a leeward corner. However, complete roof f a i l u r e is l i k e l y to occur only when connection f a i l u r e takes place on the most windward part of the building. connections would probably have f a i l e d .
At this stage, a l l other
An exception to this occurs when the
wind is blowing over a gable end. Here the windward corners go into tension first.
For this condition, the f a i l u r e mode is somewhat more speculative and
may be limited to stripping of roof sheeting or snapping of the roof ridge. The internal pressure has a significant influence on building performance. Unfortunately, i t s influence is d i f f i c u l t to define, depending upon i n t e r i o r layout, position and nature of exterior wall openings and wind direction.
For
the purposes of estimating wind speeds to cause roof f a i l u r e , two conditions are defined: (a) A secured building in which windows are protected and (b) an
P
I,
Fig. 1.
ELEVATION
12"
12'~4"
12"ELEVATION
6"
.I
J
Wind-tunnel models.
A g ~
"1
HIP ROOF
t
G"
SIDE VIEW
SIDE VIEW
-I
248
unsecured building in which windows are not protected and major windward openings are l i k e l y to occur.
In the former case, conditions are assumed to be
midway between uniform porosity (Cpi=O) and minor openings on the windward side (Cpi=O.2).
An unsecured building is assumed to have conditions halfway between
minor openings (Cpi=O.2) and major openings on the windward face (Cpi=O.6). Table 1 shows the 3 second gust wind speed, referred to the mid-height of the roof, at which the last windward corner would go into tension.
A 0° wind is
normal to the roof ridge of the building. TABLE 1 Local gust wind speeds required to produce windward tension (mph).
Roof t ~ p e
0° Wind
45° Wind
90° Wind
Secured Unsecured
Secured Unsecured
Secured Unsecured
Hip 18° Hip 27° Hip 34° Hip 45° Gable 18° Gable 27° Gable 34° Gable 45°
98 122 138 142 111 150+ 150+ 150+
64 72 78 85 68 86 104 104
80 108 150+ 150+ 68 71 150+ 140
63 69 78 150+ 54 56 64 84
140 146 126 129 68 68 72 73
88 75 77 81 54 55 57 61
In a few instances, for a l l practical purposes, the windward edge would not go into tension.
I t seemed unreasonable to extrapolate the procedure to speeds
in excess of 150 mph (67m/s) but calculated values often exceeded this value by a considerable amount. Use of the values in Table 1 would be appropriate for the prediction of f a i l u r e conditions for a roof which merely rests on the walls.
The normal
connection practice in non-hurricane areas is to use two or three nails driven at an angle through the truss or roof j o i s t into the top of the stud wall (toe-nailing).
This type of connection w i l l be referred to as Class A.
Its
ultimate capacity is highly variable but 25 lb. per foot (368N/m) of roof to wall interface is probably a reasonable average value.
With this type of
construction the values in Table I should be multiplied by approximately 1.1 to obtain the expected f a i l u r e wind speeds. In hurricane-prone areas metal plates known as hurricane anchors are often used. Normally available anchors have allowable capacities ranging for 225 lb. (l.0kN) to 520 lb. (2.31kN).
I t was assumed that the ultimate capacity of
connections using these anchors would range from 200 l b . / f t . (2.92kN/m) (Class B) to 5001b./ft. (7.30kN/m) (Class C).
To obtain f a i l u r e conditions, the
wind speeds in Table 1 should be multiplied by approximately 1.7 for Class B and by 2.4 for Class C.
249
RISK OF MAJORSTRUCTURALDAMAGETO EXISTING BUILDINGS An attempt was made to assess the v u l n e r a b i l i t y of existing buildings to damage from a typical design hurricane.
For most of the Atlantic and Gulf of
Mexico coasts of the United States t h i s is defined as a fastest mile wind speed of 100 mph (45m/s) at 33 f t .
(lOm) in open country.
This converts to a 3 second
gust of approximately 120 mph (54m/s) at the same height and in the same terrain and approximately 110 mph (4gm/s) at the mid-height of the roof. The wind-tunnel results suggested the division of buildings into two groups based on aerodynamic performance of the roofs, three groups based on type of construction and two groups based on s u s c e p t i b i l i t y to internal pressure.
To
extend the results to different types of terrain, wind pressures were adjusted in accordance with reference 3 to include sheltered (exposure B) and severe (exposure D) locations.
Table 2 describes the classifications in more d e t a i l .
Assessments of the risk of major structural damage are made in the same table. TABLE 2 Risk of major structural damage in a design hurricane.
Sheltered Secured
Class A Low
Type i Class B Class C Low Low
Class A Low
Type 2 Class B Class C Low Low
Sheltered Unsecured
Low
Low
Low
Medium
Low
Low
Open Secured
Low
Low
Low
Medium
Low
Low
Open Unsecured
High
Low
Low
High
Medium
Low
Severe Secured
Low
Low
Low
Medium
Low
Low
Severe Unsecured
High
Low
Low
High
High
Low/Medium
Type 1 Hip roofs with slopes greater than 25° Type 2 - All other roofs -
Class A Ordinary toe-nailed connections Class B - Light-duty hurricane anchors Class C Heavy-duty hurricane anchors -
-
Sheltered Wooded areas, densely packed subdivisions and centers of towns Open - Flat open country with few obstructions Severe Flat areas adjacent to the sea Secured Windows protected aginst damage, porches and carports secured against u p l i f t forces Unsecured - All other buildings with porches and carports or with windows exceeding 5% of the wall area. -
-
-
In making these assessments, consideration was given to the number of wind directions and by how much the ultimate capacity of the roof to wall connection would be exceeded.
I t must be emphasized that only the external pressures in
open terrain were actually measured. Internal pressures, capacities of connectors and the effect of shelter were all estimated.
Nevertheless, the
table does indicate very clear trends which have been borne out by observations of performance in hurricanes which approached the design level.
For example, in
Hurricane Elena i t was observed that v i r t u a l l y all buildings sheltered by trees survived.
Even in very severe exposures, buildings with steep-pitched, hip
roofs and boarded up windows survived with l i t t l e damage. Unprotected buildings with toe-nailed connections were completely destroyed and some poor quality hurricane anchors failed in houses where windows were damaged or where there were large overhangs (ref. 4). DISCUSSION I t is clear that roof shape, internal pressure and exposure have an enormous effect on the performance of roofs in severe storms. Particularly in exposed beach locations where buildings are elevated on piles to avoid flooding, the use of steep-pitched, hip roofs should be encouraged. Most building codes do not take into account shelter provided by trees and other buildings.
Clearly, this shelter has saved many an i n f e r i o r building and
some i n f e r i o r building codes as well.
Few codes in the United States stress or
even acknowledge the importance of internal pressure and many have not recognized the high suction produced on gable roofs when the wind is blowing parallel to the roof ridge. Table 1 indicates that roof f a i l u r e is not a good wind-speed indicator unless roof shape, window damage, exposure, roof connection type and wind direction are carefully observed. Observation of the loss of a roof with walls still
standing, even in open country, could indicate gust wind speeds from as
low as 50-60 mph (22-27m/s) to more than 200 mph (90m/s).
Complete destruction
of buildings has taken place in hurricanes where the measured wind speed nearby has been less than 100 mph (45m/s) at roof height, while other well constructed buildings have survived undamaged at speeds in excess of 140 mph (63m/s).
The
Fujita Scale is at best crude and possibly a misleading method of determining wind speeds in severe storms.
I t has never been calibrated, and u n t i l further
work of this type is done, i t should be used with extreme caution. Table 2, although limited in i t s scope, indicates some useful trends with regard to the necessity for upgrading the wind resistance of existing buildings. For example, a steep-pitched, hip-roof building (Type 1) appears to have a good chance of surviving the design hurricane, even with toe-nailed connections, provided that the windows are secured with plywood or storm shutters.
Any form
251
of hurricane anchor would probably enable such a building to survive even i f the windows were damaged. Type 2 roofs, however, would require hurricane anchors in a l l but the most sheltered locations.
In exposed locations, loss of windward
windows would put most buildings of this type in danger. Upgrading of this type of building could be d i f f i c u l t and expensive, but the development of natural cover would certainly improve the chances of survival. Despite the unsatisfactory performance of single-story houses in severe storms, very l i t t l e attention has been paid to understanding the problem. Unfortunately, this simple type of building has been caught in some convoluted logic.
The Fujita Scale, without any s c i e n t i f i c backing, has led people to
believe that serious damage to houses usually results from wind speeds in excess of 200mph (90m/s).
Since tornadoes are clearly linked to wind damage in the
public's mind, these poor maligned meterological events are now blamed for v i r t u a l l y a l l wind damage, even in hurricanes, and are vested with the remarkable power of avoiding buildings with roof shapes that generate l i t t l e uplift.
Severe wind damage is therefore often put down to freak wind speeds
whereas, in most cases, the cause is probably poor construction, encouraged, in some instances, by poor building codes. Typical design wind speeds in the United States, referred to the same height and averaging time as those in Table i , range from approximately 80 mph (36m/s) in the i n t e r i o r , where tornadoes are most common, to 120 mph (54m/s) on the most severe hurricane-prone coasts ( r e f . 3).
I t can be seen from Table 1 that many
gable roofs with toe-nailed connections could f a i l at speeds below the design level, even in the lowest wind-speed areas.
I f a roof were lost in these areas
at say, 60-70 mph (27-32m/s), there would probably be l i t t l e l e f t of the house after a tornado with maximumwind speeds of even 100 mph (45m/s).
I t has yet to
be proved that winds in excess of 200 mph (90m/s) exist in tornadoes, but these results suggest that serious damage, of the type often attributed to such speeds, can occur at much lower wind speeds in single-story houses as they are presently constructed in most tornado-prone areas (Class A).
I f they were
constructed to actually meet the present design wind speeds with a reasonable factor of safety, including an allowance for the effects of broken windows (Class B or C), there would be a significant reduction in tornado damage, no doubt attributed by Meterologists to an inexplicable decline in the average intensity of such storms. Construction in hurricane-prone areas has steadily improved in recent years. Performance of buildings in recent hurricanes has shown that well constructed buildings that actually meet good building code requirements can survive with l i t t l e or no damage. However, the results presented in this paper and supported by recent observations, suggest that some may be meeting the design requirements with very l i t t l e margin of safety and that i t may only take the loss of a window
252
to produce serious damage. CONCLUSIONS I t has been shown that the wind speeds at which roof failures are l i k e l y to occur in typical wood-framed houses can range from 50-60 mph (22-27m/s) to speeds far outside the range l i k e l y to be encountered in the lifetime of any building.
Even in open country, the f a i l u r e speed depends on wind direction,
roof shape, type of roof to wall connection and size and location of wall openings.
As a result, the use of damage observations to estimate wind speeds
should not be attempted unless detailed information regarding the above parameters and the degree of shelter are known. Taking into account the type of construction, amount of shelter and the protection of windows, assessments have been made regarding the risk of structural damage to one size of building during a "once in f i f t y year" wind storm on the Gulf of Mexico or Atlantic coasts of the United States.
If
extended to the other shapes of buildings this type of analysis could prove to be a valuable tool in determining the need to upgrade the wind resistance of existing buildings. Only mean external pressures were measured in the wind-tunnel tests and assessments had to be made regarding the correlation of gusts over the roofs, the nature of the load resulting from internal pressure and the ultimate capacity of roof to wall connections.
The literature contains l i t t l e
information regarding these matters and further work is required to define the d i f f e r e n t i a l pressure between the internal and external surfaces of a roof system and the actual capacity of roof to wall connections. REFERENCES 1. 2. 3. 4.
K.C. Mehta, Wind Induced Damage Observations and their Implications for Design Practice, Engineering Structures, Vol. 6, 1984. M.L. Hessig and P.R. Sparks, Wind Pressures and Forces on the Roofs of Common Residential Structures, Department of Civil Engineering Report 20S-86, Clemson University, Clemson, South Carolina, 1986. American National Standards Institute, Minimum Design Loads for Buildings and Other Structures, ANSI, New York, 1982. P.R. Sparks, J. Baker, J.D. B e l v i l l e and D.C. Perry, Hurricane Elena, Gulf Coast, August 29-September 2, 1985, National Academy Press, Washington, D.C (in press).
245
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
ON THE FAILURE OF SINGLE-STORY WOOD-FRAMEDHOUSES IN SEVERE STORMS P.R. SPARKS, M.L. HESSIG, J.A. MURDENand B.L. SILL Department of Civil Engineering, Clemson Univerity, Clemson, South Carolina 29634-0911, (USA)
ABSTRACT Wind-tunnel pressure measurements are used to define the wind forces at the roof to wall interface of typical single-story, wood-framed houses. These forces are then combined with the weight of the roof, connector capacities and the effects of internal pressure to estimate the most l i k e l y wind speeds at which a roof would separate from the walls. This is then extended to predict the risk of serious structural damage in a typical design hurricane, taking into account the form of construction, amount of shelter and the degree to which the windows are protected. Conclusions are drawn regarding the s u i t a b i l i t y of damage observations to estimate wind speeds and the s u s c e p t i b i l i t y of various forms of construction to damage in severe storms. INTRODUCTION Annual losses due to wind damage in the United States are estimated to be in excess of 3 b i l l i o n dollars (ref. 1).
A significant proportion of this damage
occurs in single-story, wood-framed houses during severe thunderstorms, tornadoes and hurricanes.
Damage is so common that Meteorologists have
developed a scale, the Fujita Scale, which classifies the extent of damage and then estimates wind speeds from that c l a s s i f i c a t i o n .
Unfortunately, t h i s scale
has been used extensively without ever being properly calibrated.
The
assumption is made that the amount of damage bears a linear relationship to the i n t e n s i t y of the storm, despite the fact that wood-framed dwellings are very b r i t t l e structures in which catastrophic f a i l u r e s often occur shortly after the loss of a roof. Modern roof construction, using trusses and plywood, usually produces a very sound structural unit which may separate intact, or at least in large sections, from the walls.
The c r i t i c a l connection, upon which the survival of the
complete structure may depend, is therefore that between the roof trusses or j o i s t s and the stud-walls. This paper describes the use of wind-tunnel test data to determine the roof-wall interface forces on a set of typical single-story houses with a variety of roof shapes, each 24 f t . by 48 f t .
(7.3m x 14.6m) in plan with 10 f t .
(3.0m) high walls and 2 f t . (O.6m) eaves. This information is then used to 0167-6105/88/$03.50
© 1988 Elsevier Science Publishers B.V.
246
assess the wind speeds at which roof failures are l i k e l y to occur and to determine the v u l n e r a b i l i t y of existing buildings to a typical design hurricane. WIND-TUNNEL TESTS These tests were conducted in the Boundary Layer Wind Tunnel at Clemson University.
Since most damagetakes place in buildings located in open country,
a boundary layer was developed to model this situation.
The mean velocity
p r o f i l e had a power law exponent of 0.16 and the longitudinal turbulence intensity was 18%. Using 1:48 scale models with typical roof slopes of 4:12 (18°), 6:12 (27°), 8:12 (34° ) and 12:12 (45° ) (Fig. 1), pressures were measured at 36 locations on the gable roofs and 54 locations on the hip roofs.
Since the turbulence
characteristics of most severe storms are not well established, i t was decided for this investigation not to attempt to use a sophisticated simultaneous pressure measurement technique.
Instead, the more t r a d i t i o n a l approach of
measuring mean values and combining with gust wind speeds, averaged over an appropriate time (3 seconds), was adopted. Outputs from pressure transducers were recorded on a microcomputer and then converted to force and moment coefficients with respect to axes passing through the center of the plane of intersection of the walls and the roof.
Further details of the wind-tunnel
tests can be found in reference 2. WIND CONDITIONS REQUIREDTO PRODUCEROOFFAILURES The results from the wind-tunnel tests were combined with roof weight and the forces resulting from internal pressure to determine roof-wall interface forces.
In doing this i t was assumed that connections would only be made on the
exterior walls parallel to the roof ridge.
This was appropriate for gable roofs
but s l i g h t l y conservative for hip roofs where some connections would be made on the end walls. For most roofs the greatest interface forces occurred at a leeward corner. However, complete roof f a i l u r e is l i k e l y to occur only when connection f a i l u r e takes place on the most windward part of the building. connections would probably have f a i l e d .
At this stage, a l l other
An exception to this occurs when the
wind is blowing over a gable end. Here the windward corners go into tension first.
For this condition, the f a i l u r e mode is somewhat more speculative and
may be limited to stripping of roof sheeting or snapping of the roof ridge. The internal pressure has a significant influence on building performance. Unfortunately, i t s influence is d i f f i c u l t to define, depending upon i n t e r i o r layout, position and nature of exterior wall openings and wind direction.
For
the purposes of estimating wind speeds to cause roof f a i l u r e , two conditions are defined: (a) A secured building in which windows are protected and (b) an
P
I,
Fig. 1.
ELEVATION
12"
12'~4"
12"ELEVATION
6"
.I
J
Wind-tunnel models.
A g ~
"1
HIP ROOF
t
G"
SIDE VIEW
SIDE VIEW
-I
248
unsecured building in which windows are not protected and major windward openings are l i k e l y to occur.
In the former case, conditions are assumed to be
midway between uniform porosity (Cpi=O) and minor openings on the windward side (Cpi=O.2).
An unsecured building is assumed to have conditions halfway between
minor openings (Cpi=O.2) and major openings on the windward face (Cpi=O.6). Table 1 shows the 3 second gust wind speed, referred to the mid-height of the roof, at which the last windward corner would go into tension.
A 0° wind is
normal to the roof ridge of the building. TABLE 1 Local gust wind speeds required to produce windward tension (mph).
Roof t ~ p e
0° Wind
45° Wind
90° Wind
Secured Unsecured
Secured Unsecured
Secured Unsecured
Hip 18° Hip 27° Hip 34° Hip 45° Gable 18° Gable 27° Gable 34° Gable 45°
98 122 138 142 111 150+ 150+ 150+
64 72 78 85 68 86 104 104
80 108 150+ 150+ 68 71 150+ 140
63 69 78 150+ 54 56 64 84
140 146 126 129 68 68 72 73
88 75 77 81 54 55 57 61
In a few instances, for a l l practical purposes, the windward edge would not go into tension.
I t seemed unreasonable to extrapolate the procedure to speeds
in excess of 150 mph (67m/s) but calculated values often exceeded this value by a considerable amount. Use of the values in Table 1 would be appropriate for the prediction of f a i l u r e conditions for a roof which merely rests on the walls.
The normal
connection practice in non-hurricane areas is to use two or three nails driven at an angle through the truss or roof j o i s t into the top of the stud wall (toe-nailing).
This type of connection w i l l be referred to as Class A.
Its
ultimate capacity is highly variable but 25 lb. per foot (368N/m) of roof to wall interface is probably a reasonable average value.
With this type of
construction the values in Table I should be multiplied by approximately 1.1 to obtain the expected f a i l u r e wind speeds. In hurricane-prone areas metal plates known as hurricane anchors are often used. Normally available anchors have allowable capacities ranging for 225 lb. (l.0kN) to 520 lb. (2.31kN).
I t was assumed that the ultimate capacity of
connections using these anchors would range from 200 l b . / f t . (2.92kN/m) (Class B) to 5001b./ft. (7.30kN/m) (Class C).
To obtain f a i l u r e conditions, the
wind speeds in Table 1 should be multiplied by approximately 1.7 for Class B and by 2.4 for Class C.
249
RISK OF MAJORSTRUCTURALDAMAGETO EXISTING BUILDINGS An attempt was made to assess the v u l n e r a b i l i t y of existing buildings to damage from a typical design hurricane.
For most of the Atlantic and Gulf of
Mexico coasts of the United States t h i s is defined as a fastest mile wind speed of 100 mph (45m/s) at 33 f t .
(lOm) in open country.
This converts to a 3 second
gust of approximately 120 mph (54m/s) at the same height and in the same terrain and approximately 110 mph (4gm/s) at the mid-height of the roof. The wind-tunnel results suggested the division of buildings into two groups based on aerodynamic performance of the roofs, three groups based on type of construction and two groups based on s u s c e p t i b i l i t y to internal pressure.
To
extend the results to different types of terrain, wind pressures were adjusted in accordance with reference 3 to include sheltered (exposure B) and severe (exposure D) locations.
Table 2 describes the classifications in more d e t a i l .
Assessments of the risk of major structural damage are made in the same table. TABLE 2 Risk of major structural damage in a design hurricane.
Sheltered Secured
Class A Low
Type i Class B Class C Low Low
Class A Low
Type 2 Class B Class C Low Low
Sheltered Unsecured
Low
Low
Low
Medium
Low
Low
Open Secured
Low
Low
Low
Medium
Low
Low
Open Unsecured
High
Low
Low
High
Medium
Low
Severe Secured
Low
Low
Low
Medium
Low
Low
Severe Unsecured
High
Low
Low
High
High
Low/Medium
Type 1 Hip roofs with slopes greater than 25° Type 2 - All other roofs -
Class A Ordinary toe-nailed connections Class B - Light-duty hurricane anchors Class C Heavy-duty hurricane anchors -
-
Sheltered Wooded areas, densely packed subdivisions and centers of towns Open - Flat open country with few obstructions Severe Flat areas adjacent to the sea Secured Windows protected aginst damage, porches and carports secured against u p l i f t forces Unsecured - All other buildings with porches and carports or with windows exceeding 5% of the wall area. -
-
-
In making these assessments, consideration was given to the number of wind directions and by how much the ultimate capacity of the roof to wall connection would be exceeded.
I t must be emphasized that only the external pressures in
open terrain were actually measured. Internal pressures, capacities of connectors and the effect of shelter were all estimated.
Nevertheless, the
table does indicate very clear trends which have been borne out by observations of performance in hurricanes which approached the design level.
For example, in
Hurricane Elena i t was observed that v i r t u a l l y all buildings sheltered by trees survived.
Even in very severe exposures, buildings with steep-pitched, hip
roofs and boarded up windows survived with l i t t l e damage. Unprotected buildings with toe-nailed connections were completely destroyed and some poor quality hurricane anchors failed in houses where windows were damaged or where there were large overhangs (ref. 4). DISCUSSION I t is clear that roof shape, internal pressure and exposure have an enormous effect on the performance of roofs in severe storms. Particularly in exposed beach locations where buildings are elevated on piles to avoid flooding, the use of steep-pitched, hip roofs should be encouraged. Most building codes do not take into account shelter provided by trees and other buildings.
Clearly, this shelter has saved many an i n f e r i o r building and
some i n f e r i o r building codes as well.
Few codes in the United States stress or
even acknowledge the importance of internal pressure and many have not recognized the high suction produced on gable roofs when the wind is blowing parallel to the roof ridge. Table 1 indicates that roof f a i l u r e is not a good wind-speed indicator unless roof shape, window damage, exposure, roof connection type and wind direction are carefully observed. Observation of the loss of a roof with walls still
standing, even in open country, could indicate gust wind speeds from as
low as 50-60 mph (22-27m/s) to more than 200 mph (90m/s).
Complete destruction
of buildings has taken place in hurricanes where the measured wind speed nearby has been less than 100 mph (45m/s) at roof height, while other well constructed buildings have survived undamaged at speeds in excess of 140 mph (63m/s).
The
Fujita Scale is at best crude and possibly a misleading method of determining wind speeds in severe storms.
I t has never been calibrated, and u n t i l further
work of this type is done, i t should be used with extreme caution. Table 2, although limited in i t s scope, indicates some useful trends with regard to the necessity for upgrading the wind resistance of existing buildings. For example, a steep-pitched, hip-roof building (Type 1) appears to have a good chance of surviving the design hurricane, even with toe-nailed connections, provided that the windows are secured with plywood or storm shutters.
Any form
251
of hurricane anchor would probably enable such a building to survive even i f the windows were damaged. Type 2 roofs, however, would require hurricane anchors in a l l but the most sheltered locations.
In exposed locations, loss of windward
windows would put most buildings of this type in danger. Upgrading of this type of building could be d i f f i c u l t and expensive, but the development of natural cover would certainly improve the chances of survival. Despite the unsatisfactory performance of single-story houses in severe storms, very l i t t l e attention has been paid to understanding the problem. Unfortunately, this simple type of building has been caught in some convoluted logic.
The Fujita Scale, without any s c i e n t i f i c backing, has led people to
believe that serious damage to houses usually results from wind speeds in excess of 200mph (90m/s).
Since tornadoes are clearly linked to wind damage in the
public's mind, these poor maligned meterological events are now blamed for v i r t u a l l y a l l wind damage, even in hurricanes, and are vested with the remarkable power of avoiding buildings with roof shapes that generate l i t t l e uplift.
Severe wind damage is therefore often put down to freak wind speeds
whereas, in most cases, the cause is probably poor construction, encouraged, in some instances, by poor building codes. Typical design wind speeds in the United States, referred to the same height and averaging time as those in Table i , range from approximately 80 mph (36m/s) in the i n t e r i o r , where tornadoes are most common, to 120 mph (54m/s) on the most severe hurricane-prone coasts ( r e f . 3).
I t can be seen from Table 1 that many
gable roofs with toe-nailed connections could f a i l at speeds below the design level, even in the lowest wind-speed areas.
I f a roof were lost in these areas
at say, 60-70 mph (27-32m/s), there would probably be l i t t l e l e f t of the house after a tornado with maximumwind speeds of even 100 mph (45m/s).
I t has yet to
be proved that winds in excess of 200 mph (90m/s) exist in tornadoes, but these results suggest that serious damage, of the type often attributed to such speeds, can occur at much lower wind speeds in single-story houses as they are presently constructed in most tornado-prone areas (Class A).
I f they were
constructed to actually meet the present design wind speeds with a reasonable factor of safety, including an allowance for the effects of broken windows (Class B or C), there would be a significant reduction in tornado damage, no doubt attributed by Meterologists to an inexplicable decline in the average intensity of such storms. Construction in hurricane-prone areas has steadily improved in recent years. Performance of buildings in recent hurricanes has shown that well constructed buildings that actually meet good building code requirements can survive with l i t t l e or no damage. However, the results presented in this paper and supported by recent observations, suggest that some may be meeting the design requirements with very l i t t l e margin of safety and that i t may only take the loss of a window
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to produce serious damage. CONCLUSIONS I t has been shown that the wind speeds at which roof failures are l i k e l y to occur in typical wood-framed houses can range from 50-60 mph (22-27m/s) to speeds far outside the range l i k e l y to be encountered in the lifetime of any building.
Even in open country, the f a i l u r e speed depends on wind direction,
roof shape, type of roof to wall connection and size and location of wall openings.
As a result, the use of damage observations to estimate wind speeds
should not be attempted unless detailed information regarding the above parameters and the degree of shelter are known. Taking into account the type of construction, amount of shelter and the protection of windows, assessments have been made regarding the risk of structural damage to one size of building during a "once in f i f t y year" wind storm on the Gulf of Mexico or Atlantic coasts of the United States.
If
extended to the other shapes of buildings this type of analysis could prove to be a valuable tool in determining the need to upgrade the wind resistance of existing buildings. Only mean external pressures were measured in the wind-tunnel tests and assessments had to be made regarding the correlation of gusts over the roofs, the nature of the load resulting from internal pressure and the ultimate capacity of roof to wall connections.
The literature contains l i t t l e
information regarding these matters and further work is required to define the d i f f e r e n t i a l pressure between the internal and external surfaces of a roof system and the actual capacity of roof to wall connections. REFERENCES 1. 2. 3. 4.
K.C. Mehta, Wind Induced Damage Observations and their Implications for Design Practice, Engineering Structures, Vol. 6, 1984. M.L. Hessig and P.R. Sparks, Wind Pressures and Forces on the Roofs of Common Residential Structures, Department of Civil Engineering Report 20S-86, Clemson University, Clemson, South Carolina, 1986. American National Standards Institute, Minimum Design Loads for Buildings and Other Structures, ANSI, New York, 1982. P.R. Sparks, J. Baker, J.D. B e l v i l l e and D.C. Perry, Hurricane Elena, Gulf Coast, August 29-September 2, 1985, National Academy Press, Washington, D.C (in press).